In the operation of commercial and industrial processes, it is common to produce a dispersion or slurry of solid particles in a liquid in which they float. Furthermore, it is often necessary or desirable to separate the solid particles from the liquid. One such dispersion or slurry which requires such separation is an aqueous liquid slurry of ice particles or crystals.
Various freeze processes have been developed to produce potable water from seawater, brackish water, or industrial waste waters; to concentrate fruit juices such as orange juice and grape juice, vegetable juices such as tomato juice; coffee; and to separate dissolved or suspended salts from the liquid carrier. See, for example, U.S. Pat. Nos. 3,070,969; 3,477,241; 3,501,924; 3,620,034; 3,664,145; and 4,091,635.
Many types of equipment and heat exchangers have been used in the described freeze concentration processes, some of which is disclosed in the patents listed above. More recently, shell and tube freeze exchangers have been developed for this purpose as disclosed in U.S. Pat. Nos. 4,286,436 and 4,335,581.
In producing a freeze concentrated aqueous product using a shell and tube vertical freeze exchanger, the product to be concentrated is generally precooled and then is fed into the top of the tubes. As it flows downwardly through the vertical tubes, it is further cooled by heat exchange with a cold fluid circulated through the shell side of the heat exchanger. The cold fluid is generally a refrigerant such as ammonia or a Freon brand refrigerant. As the aqueous liquid cools, ice crystals form. The mixture of cold aqueous liquid flows from the tubes into a receiving tank in which the ice separates as a slurry floating on concentrated aqueous liquid. Some of the concentrated aqueous liquid is generally recycled to the freeze exchanger to produce more ice, while the ice slurry is removed and washed to recover any product, such as a juice, on the ice. Of course, if potable water is desired, the ice is washed and then melted. The concentrated aqueous liquid in the receiving tank, for example, a juice, can be withdrawn and packaged.
Apparatus for separating and washing an ice slurry for the described purposes have been known in the art for a considerable period of time. One type of such apparatus is referred to as a gravity counterwasher. In this type of separator and washer, an ice slurry is introduced at the base of a vertical, cylindrical vessel. The buoyancy of the ice and the upward flow of the aqueous liquid cause the ice particles to rise, where they form a bed or pack. The aqueous liquid passes through the lower portion of the pack and discharges through drains located near the midpoint of the vessel. A pressure drop created by the liquid flow through the base of the ice pack causes the pack to rise, much like a piston in a cylinder. Fresh water is sprayed onto the top of the pack, displacing the concentrated aqueous liquid (the brine), thus washing the uppermost crystals. Cleaned ice is continuously scraped from the top of the ice pack. Such an apparatus is disclosed in U.S. Pat. No. 4,341,085.
Although counterwashers of the described type have seen considerable use, problems are encountered with their operation. To be functional, wash water must penetrate the top of the pack. The rate of penetration is limited by the acceleration of gravity and the flow resistance of the pack. This results in very large counterwashers, thereby raising capital costs, lengthening startup times and increasing heat transfer to the surroundings. Furthermore, significant wash water losses occur since some wash water travels through the pack directly to the drain. These losses are much greater than losses due to diffusion between wash water and concentrated liquid.
Operation of a counterwasher also requires a variation in pressure through the pack. This can result in non-uniform compression forces and high compressive loads in the region of the pack below the concentrated liquid drains. The permeability of ice decreases when the ice is subjected to high compression. The resulting low permeability can limit wash water penetration and can make column operation difficult. In addition, the high flow rate through this part of the pack can cause channel formation. When this occurs, slurry passes directly through the drains, there is no separation of ice and concentrated liquid, and the pack stops moving upward.
It is also recognized that counterwasher operating controls are limited to inlet and outlet flow parameters. Also, the height of the fresh water zone is very important to column operation. Small upsets in the rate of wash water drainage can change this height and have harmful effects on column operation and product quality. Successful, repeatable and optimal operation of a large scale counterwasher can be difficult to achieve.
From the above discussion it is believed clear that alternative apparatus and methods for separating solid particles from a liquid in which the particles float and, if desired, washing the separated solid particles would be useful, especially in separating ice crystals from an aqueous dispersion or slurry.